The present invention relates to a semiconductor device including an insulating substrate, a plurality of pixel electrodes arranged in a matrix on the insulating substrate, switching elements for individually driving the pixel electrodes, and driving circuits formed on the insulating substrate for operating the switching elements, and particularly to a structure of a bottom-gate type thin film transistor constituting each of the switching elements and the driving circuits.
Semiconductor devices each having integrated thin film transistors and the like are particularly suitable for driving substrates of active matrix type electro-optical devices, and therefore, they are being extensively developed at present. A thin film transistor has a semiconducting thin film as an active layer, which is made from amorphous silicon or polycrystalline silicon. The polycrystalline silicon transistor is superior in electric characteristics such as a carrier mobility to the amorphous silicon transistor, and it can be used for a peripheral driving circuit as well as for a switching element. In this regard, studies are being actively conducted on the polycrystalline transistors. On the other hand, when used for an active matrix display which is one example of the active matrix type electro-optical devices, the semiconductor device must adopt an inexpensive large-sized insulating substrate. From this viewpoint, there is a strong demand to develop a low temperature process capable of forming thin film transistors at a temperature in a range of 600.degree. C. or less, preferably, 400.degree. C. or less. Laser annealing or ion doping becomes important for the low temperature process. Moreover, from the standpoint of the structure, the thin film transistor is classified into a bottom-gate type (reversely staggered type) and a top-gate type. The bottom-gate type is superior to the top-gate type in terms of compatibility with the low temperature process, which has been proposed, for example, in Japanese Patent Laid-open Nos. Hei 4-186735 and Hei 6-350089.
In summary, the bottom-gate type polycrystalline silicon thin film transistor is regarded hopeful in that it can be formed by the low temperature process and it can constitute a peripheral driving circuit. One example of the thin film transistors of this type will be described with reference to FIG. 8. As shown in this figure, a bottom-gate type thin film transistor 100 includes a gate electrode 102 made from a metal film patterned on an insulating substrate 101 made from glass or the like, an gate insulating film 103 covering the gate electrode 102, and a semiconducting thin film 104 formed on the gate insulating film 103. The semiconducting thin film 104 is made from, for example, polycrystalline silicon. A channel region 105 and a highly doped region 106 for forming a drain D and a source S are formed in the semiconducting thin film 104. A first interlayer insulating film 107 serving as a stopper is formed on the channel region 105, and a second interlayer insulating film 108 is formed thereon. The first interlayer insulating film 107 and the second interlayer insulating film 108 may be replaced with a single layer structure. In each case, such an interlayer insulating film, disposed opposite to the gate insulating film 103, is called "back gate insulating film". The second interlayer insulating film 108 has contact holes, through which interconnection electrodes 109 are connected to the source S and the drain D of the thin film transistor 100.
In the structure of the bottom-gate type thin film transistor 100 shown in FIG. 8, the gate electrode 102 is formed on the insulating substrate 101 side and the gate insulating film 103 and the semiconducting thin film 104 are formed on the gate electrode 102, and the interconnection electrodes 109 are formed in such a manner as to be connected to the semiconducting thin film 104. In the case where a semiconductor device having the integrated thin film transistors 100 is applied to a large-sized active matrix display, it is important uses an insulating substrate made from inexpensive glass, and accordingly, it must be fabricated in accordance with a low temperature process performed at a temperature of 400.degree. C. or less. In this case, however, there arises a problem that the film density is coarsened with respect to the interlayer insulating films 107, 108, the gate insulting film 103, and the semiconducting thin film 104 because each film is formed at a low temperature. As a result, the local level density of each film is increased, and thereby charges flowing in the channel region 105 of the thin film transistor 100 in a driving state are injected in an interface between the semiconducting thin film 104 and the insulating film or in the insulating film. In other words, hot carriers are injected into the back gate insulating film and the gate insulating film formed from the interlayer insulating film by impact ionization occurring at the drain edge in a high electric field condition. It is possible that the stationary charges thus injected shift a threshold voltage of the thin film transistor 100, to cause an operation of the transistor even not in the on-state, leading to a malfunction of the peripheral driving circuit.
The problem to be solved will be further described in brief with reference to FIG. 9. The gate insulating film 103, and the interlayer insulating films 107 and 108 located near the back gate contain impurities and migratory ions. This is due to formation of the source S and the drain D by ion doping which is adopted for achieving the low temperature process. Specifically, the ion doping is performed such that an ionized source gas is injected in the semiconducting thin film 104 acceleratedly in the presence of an electric field without mass-separation. As a result, the ion shower contains other impurities than the kinds of targets, and thereby migratory ions and the like are injected in the interlayer insulating films 107 and 108. In the case where a high electric field is applied between the source S and the drain D in such a state, the migratory ions migrate from the drain edge to the source edge. With an N-channel type thin film transistor, the polarity of a portion near the drain edge is changed from N.sup.+ into P.sup.+ by migration of the migratory ions, to cause current limiting, thereby making it difficult to obtain a sufficient driving current. In particular, when used for a peripheral driving circuit, the thin film transistor having the above configuration arises a problem in degradation of high speed operation. As described above, in the case of the bottom-gate type thin film transistor prepared by the low temperature process, each of the gate insulating film and the interlayer insulating film respectively formed on upper and lower sides of the semiconducting thin film made from polycrystalline silicon or the like must be constituted of a deposition film formed by CVD or the like, tending to be increased in interfacial level density and impurity concentration. Consequently, charges are easily injected in the insulating film by impact ionization under a high electric field applied between the drain and source, and migratory ions acting as undesirable impurities in the insulating film degrade transistor characteristics.